--- a/python/mozboot/mozboot/base.py
+++ b/python/mozboot/mozboot/base.py
@@ -284,17 +284,17 @@ class BaseBootstrapper(object):
             if e.errno == errno.EEXIST and os.path.isdir(download_path):
                 pass
             else:
                 raise
 
         try:
             package_url = TOOLTOOL_API + package_sha512sum
             downloaded_filename = os.path.join(download_path, package_sha512sum)
-            print('Downloading clang package from %s', package_url)
+            print('Downloading clang package from %s' % package_url)
             self.http_download_and_save(package_url, downloaded_filename,
                                         package_sha512sum, 'sha512')
 
             # We don't have to handle a great variety of archive types here.
             if package_filename.endswith('tar.gz'):
                 cmd = ['tar', 'zxf', downloaded_filename]
             elif package_filename.endswith('.tar.bz2'):
                 cmd = ['tar', 'jxf', downloaded_filename]